Advances in semiconductor device performance are largely attributable to reductions in device dimensions. Initially, the photolithographic and other micro-electronic fabrication techniques enabling such device scaling were not amenable to inductors, capacitors and other passive components often employed in semiconductor devices. Nevertheless, inductor design eventually evolved to take similar advantage of the advances in micro-electronic fabrication processes and experience scaling to a degree similar to that of active devices.
For example, inductors are now typically created on the surface of a semiconductor device substrate in a spiral shape created in a plane that is parallel to the substrate surface. The inductor may have several of these spiral-shaped coils as necessary to attain adequate inductance values. Unfortunately, conventional methods used to create the spiral-shaped inductors generally produce inductor devices having limited performance, as discussed below.
The performance parameter of an inductor is typically indicated as its quality factor Q. The quality factor is defined as the ratio between the energy stored in the reactive portion of the inductor and the energy that is lost in the reactive portion. Clearly, it is desired that the quality factor be maximized to the extent permitted within a particular application.
The Q value of an inductor can also be expressed by the equation Q=W0L/R, where W0 is the resonant frequency of oscillation of the inductor, L is the inductive value and R is the resistance of the inductor. This equation further indicates that, for a given resonant frequency W0, the Q value of the inductor decreases as the resistance R increases.
However, current fabrication techniques or other factors limit the degree to which the resistance of existing inductor devices can be reduced. Consequently, inductor devices typically incorporated in conventional integrated circuits exhibit a quality factor limited to about 10.
Accordingly, what is needed in the art is an inductor device that addresses the problems discussed above.